The present invention generally relates to hydroelectric plants arranged in series along a watercourse. The invention more precisely relates to a system for driving a turbined water flow rate of a plurality of hydroelectric plants arranged in series along a watercourse, as well as a plurality of hydroelectric plants driven by said driving system.
Turbined water designates the water passing through a hydraulic turbine in a fall in order to produce mechanical energy which is then converted into electrical energy. Thus, the electric power generated can be deduced from the turbined water flow rate, and vice-versa, by means of conversion charts.
The situation in which hydroelectric plants are arranged in series along a watercourse is a common situation, since it corresponds for example to cases in which several power generation hydraulic structures are arranged along a river or a tributary, for example in a valley.
Such a situation is schematised in FIG. 1, which is a scheme illustrating a non-limiting example of the arrangement of five hydroelectric plants arranged in series along a watercourse defining upstream of the same a plurality of corresponding reaches.
In this scheme, the plants and reaches are numbered in the order of their arrangement along the watercourse, from upstream to downstream, the water circulation direction being noted by an arrow. Thus, for example, plant U3 has plants U1 and U2 as upstream plants, and plants U4 and U5 as downstream plants.
Likewise, reach 3 surrounded by plant U3 and plant U2, has as its upstream reaches reach 1 upstream of plant U1 and reach 2 between plant U1 and plant U2, and has as its downstream reaches, reaches 4 and 5 respectively defined between plant U3 and plant U4, and between plant U4 and plant U5. Each reach i is defined by the portion of the watercourse immediately upstream of plant Ui, that is between plant Ui−1 and plant Ui, and is in particular characterised by a water level.
These plants are arranged “run-of-the-river” along the watercourse, typically a watercourse such as a river, a tributary or a canal. Thus, water arrives from reach 1 upstream of the plurality of plants first passes through the first plant U1 to reach the second reach 2, and then passes through the second plant U2 to reach the third reach 3, . . . up to pass through the last plant U5 and leave the plurality of reaches, for example to join the watercourse L downstream of the plurality of plants, or even a sea.
The following description will be purely illustrative and in no way limiting in reference to such a configuration. Further, for the sake of clarity and concision, in the rest of the description, the index i will be referred to as a general reference of an ordinal index. Thus, i can designate 1, 2, 3, 4 or 5, as well as plant Ui will designate plant U1, plant U2, plant U3, plant U4 or plant U5.
These run-of-the-river hydroelectric plants are installed in cascade along a river or a tributary and are exploited with the main objective, besides power generation, not to disturb the natural flow of the watercourse used for multiple uses (maritime navigation, agricultural withdrawals, nautical activities . . . ): the aim is mainly to convey flow rate from upstream to downstream by fulfilling flow rate (amplitude and variation) and level hydraulic requirements, that is maintaining in each of the reaches i, the water level within acceptable limits. To that end, the plants are equipped with a level regulation which modifies the flow rate setpoint of the facility in order to regulate the level.
Run-of-the-river plants have an unavoidable power generation difficult to foresee which undergoes variation of the uncontrolled inflow rate Qe as well as unforeseen supplies/withdrawals in the reaches i, such as the tributary flow rate Qa3 feeding the reach 3 in FIG. 1. That results in resorting to a level regulation of the reaches i, modifying the power produced by the plant and thus de-optimising the production program designed by the optimiser (in charge of optimising the production on the entire facility covered by the producer) the day before. Indeed, an underproduction of the queue created by a hydraulic disturbance has to be compensated for by starting another production means which has a cost.
In view of the unforeseen hydraulic events (catchment area draining, rain, withdrawals in a reach, turbine variation of the upstream plant) that can occur upstream of a chain of run-of-the-river plants, fulfilling a daily production program is difficult and the modifications of the production program causing redefinitions of the power setpoint value applied to the series of plants, are detrimental to the fulfillment of a production program.
Furthermore, hydroelectric plants are increasingly resorted to contribute in real time to the production-consumption balance of the grid by providing a frequency regulation service (commonly called “Ancillary Service”). It consists in varying in real time the provided power about its operating point by following the variations in the frequency of the power grid (it is called a frequency-power primary regulation) and/or the evolution of the remote control level N (it is called a frequency-power secondary regulation).
The frequency-power secondary regulation service given to the grid by the producer is particularly difficult to reconcile with hydraulic requirements: the variations in turbined flow rate to provide this service disturb the water level in the reaches which is controlled by the level regulation. In the absence of a particular device, this level regulation naturally counterbalances the frequency-power secondary regulation. Reconciling this frequency-power secondary regulation and the fulfillment of the hydraulic requirements in the reaches is thus clearly difficult.
FIG. 2 is a scheme illustrating a system for driving a plurality of hydroelectric plants arranged in series along a watercourse. For the sake of clarity and concision, three plants are illustrated: a plant Ui, a plant Ui−1 upstream of plant Ui, and plant Ui+1 downstream of plant Ui.
Conventionally, run-of-the-river hydroelectric plants are managed in a flow rate with a flow rate setpoint on plant Ui (QCui) equal to the run-of-the-river flow rate setpoint QFi. For run-of-the-river hydroelectric plants, the objective is traditionally to mandatorily fulfil the hydraulic (flow rate and level) requirements for the different reaches i and to respect as much as possible a power program defined for 24 h by the optimiser. The control on the water flow rate Qcui turbined by plant Ui is decomposed into the sum of three control terms.
A first term is the parallel anticipation flow rate QAPi, which gathers the sum of the natural inflow rates measured upstream of reach i and in the same. Therefore, it is the sum of the inflow rate Qe from upstream of the series of plants, that is the flow rate arriving at the first reach 1, to which the flow rates Qai of the possible tributaries of each reach i are added. Thus:
      QAP    i    =      Qe    +                  ∑                  k          =          1                i            ⁢                          ⁢              Qa        k            
The second term, noted QCHi, is a level regulation flow rate of the reach i. Traditionally, the equivalent level zeqi of reach i is controlled by plant Ui only by means of the level regulation, of the proportional-integral (PI) corrector type controlling the level of reach i to a level setpoint Zci, which depends in particular on the hydraulic requirements of reach i. The equivalent level zeqi corresponds to the weighting of several levels along reach i and is representative of the water volume in reach i.
A third term is usually a series anticipation flow rate QASi relating to the level regulation of the reaches, taking into account the difference between the setpoint Qcui−1 and the parallel anticipation flow rate QAPi−1 of the plant Ui−1 upstream of said plant Ui:QASi=QCui−1−QAPi−1 
It is to be noted that generally the first upstream plant U1 does not have a series anticipation available, QAS1=0. It can be further shown that the series anticipation flow rate QASi corresponds to the sum of the level regulation flow rate QCH of all the reaches upstream of said plant Ui, if no saturation or ramp limitation on the flow rates is activated, such that the series anticipation flow rates QASi for a plant Ui can also be written for i≥2 as:
      QAS    i    =            ∑              k        =        1            i        ⁢                  ⁢          QCH      k      
with QAS1=0.
Thus, there isQCui=QAPi+QCHi+QASi 
Insofar as these flow rate control terms aim at maintaining the level constant in the reaches outside a power regulation and outside a demodulation, they can be gathered as run-of-the-river flow rate setpoint QFi:QCui=QFi=QAPi+QCHi+QASi This driving system generally allows the levels of the reaches i to be regulated by modifying the turbined water flow rates by the hydroelectric plants Ui, so as to ensure hydraulic safety in any situation. Hydraulic safety relates in particular to the fulfillment of level requirements in the reaches. These level requirements are tidal range requirements: minimum and maximum levels permitted for each reach in order to allow navigation and ensure safety of goods and people. Tidal range is by definition low for run-of-the-river plants and is often null for the most upstream plant. The exploitation of series run-of-the-river plants is all the more difficult that the tidal ranges permitted are low. Further, other hydraulic requirements make this driving difficult. Thus, the inflow rate Qe is uncontrolled, and can strongly vary, resulting for example from the turbining of an upstream electrical producer, and/or upstream catchment area supplies (rain, snow melting . . . ).
It is also imposed to release, at the output of the series of plants, the inflow rate Qe, possibly added with the flow rates Qai of the tributaries of the reaches. It is called demodulation. The outflow rate turbined by the last plant is thus required to be “demodulated”. The conventional demodulation consists in releasing at the end of the chain the sum of the natural inflow rates measured upstream in order to cancel all the disturbances generated by the operation of the hydroelectric plants of the queue. These are however measured flow rates, that is marred by measurement errors; yet, the level regulations equipping the plants eliminate these measurement errors by turbining the real inflow rate.
The demodulation criterion to be fulfilled on the last hydroelectric plant n is thus
      Qt    n    =                    QAP        n            +      μ        =          Qe      +                        ∑                      k            =            1                    n                ⁢                                  ⁢                  Qa          k                    +      μ      where Qtn designates the flow rate turbined by plant n and μ is a permissible tolerance, in the order 2% with respect to the inflow rate in the queue Qe. It is to be noted that this demodulation criterion is a strong requirement difficult to reconcile for common controls in other fields, and which thus prevents them from being used to control a plurality of hydroelectric plants arranged run of the river.
Within the scope of this driving, from the forecast the day before for the inflow rate Qe averaged over 24 hours, a production program is set by the producer optimiser. In theory, with this flow rate driving mode, the inflow rates in the queue are integrally released downstream, the water levels in the reaches are maintained at a constant height.
The goal of the operator of the series of plants is to fulfil as long as possible the power production program demanded by the optimiser while fulfilling the exploitation hydraulic (flow rate and level) requirements. The random character of the inflow rate Qe which can abruptly vary following a load variation of the upstream plant however makes this power generation program follow-up difficult.
However, if operating requirements (typically an inflow rate which substantially differs from that which has been used by the hydraulic producer to design the power setpoint) cause a risk of non-fulfillment of the hydraulic requirements (typically a drift in the reach levels), the operators naturally modify the power setpoint by “restating” the value to the grid manager. But, this “restatement” generates significant financial penalties attributed to the producer. It is thus desirable to minimise the number of daily restatements.
Thus, conventional systems for driving plants in flow rate has several drawbacks:                the daily production program is only fulfilled with some tolerance and under several produced power prediction restatements,        providing the frequency-power secondary regulation with the required dynamic performance is not possible,        the demodulation at the level of the turbined flow rate on the last plant is only ensured with manual corrections in the plant flow rate setpoint QCun.        
Further, no power regulation loop exists to hold the overall power of the series of plants. It means that in case of unpredicted disturbance in the inflow rates of the reaches I, a deviation is observed in the electric power made relative to the electric power programmed for the series of hydroelectric plants.